We report structure refinements of three ferromagnesian cummingtonite crystals with Mg/(Mg + Fe) ≈ 0.63. An unheated crystal with XMgM4=0.051 has C2/m symmetry, but crystals heat treated at 600 and 700 °C, with XMgM4=0.167 and 0.237, respectively, have P21/m symmetry. In P21/m cummingtonite from this study and from Hirschmann et al. (1994), the A silicate chain is S rotated and the B chain is O rotated. This differs from the structure of P21/m manganoan cummingtonite, in which both A and B chains are O rotated (Papike et al., 1969). Documented in nature for the first time, the S-rotated silicate chains in amphibole suggest that, like the P21/c to C2/c phase transition in clinopyroxene, the change of the A-chain configuration from S to O rotation is intrinsic to the P21/m to C2/m transformation in ferromagnesian cummingtonite.

Documentation of different space groups for magnesio-cummingtonite crystals that differ only in site occupancies confirms that the M4-site population controls the relative stabilities of the C2/m and P21/m phases and places limits on the tolerance of the C2/m structure for Mg on the M4 site. At room temperature, ferromagnesian cummingtonite with XMgM4>0.15±0.02 has P21/m symmetry. In the P21/m structure, the separation between M4-O6A and M4-O6B and between M4-O5A and M4-O5B bond distances increases with increasing Mg content, as does the kinking angle (O5-O6-O5) of the A and B chains. Structural distortions away from C2/m symmetry, particularly in the configuration of the A chain, are greater for Fe-bearing cummingtonite than for Mn-bearing varieties at similar Mg concentration, indicating that for a given Mg content, ferromagnesian amphibole is more stable in the P21/m structure than manganoan amphibole.

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